The present invention relates to a method of manufacturing an information recording medium having a structure constituted by two disk substrates pasted together, an apparatus for manufacturing the medium, and a multi-substrate information recording medium obtained by the manufacturing method and, more particularly, to improvements in a bonding method and arrangement for a two-substrate double-layered optical disk designed such that pieces of information on two information recording layers are read from one surface of the optical disk.
Small-diameter optical disks (diameters: about five inches/120 mm to about 3.5 inches/80 mm) on which large-volume data, e.g., various types of video and speech data, can be recorded as digital data have recently been developed. According to an optical disk of this type, one or more information recording layers are formed on at least one of two disk substrates to ensure a large information storage amount. These substrates must be accurately pasted together to thereby obtain a large-capacity disk having a predetermined thickness (1.2 mm).
Assume that two disk substrates respectively having information recording layers formed thereon are accurately pasted together with an adhesive to obtain a large-capacity optical disk. When pieces of information on the internal information recording layers are read from one of the two disk substrates, a read laser invariably reciprocates via the adhesive layer between the substrates. At this time, if dust or an air bubble enters the adhesive layer between the substrates, an error occurs when reading the information corresponding to the position of the dust or the air bubble.
If the adhesive layer between the substrates is uneven and has thickness irregularity, since the laser traveling path length varies at the portions of the adhesive layer at which its thickness changes, an error tends to occur when reading the information corresponding to a position where thickness irregularity occurs.
If air bubbles enter the adhesive layer between the substrates, the strength of this adhesive layer decreases. For this reason, the disk substrates pasted together may peel away from each other upon to the impact of falling or the like.
If only one of the two disk substrates has an information recording layer, and the other substrate does not have a similar information recording layer (or a dummy layer corresponding to the recording layer), the pasted surface may deteriorate with environmental changes. As a result, the two disk substrates pasted together may peel away from each other or become deformed.
Dust or air bubbles must be completely removed, and the thickness irregularity of the adhesive layer between the substrates must be minimized (e.g., within .+-.5 .mu.m with respect to an adhesive layer thickness of 50 .mu.m). It is, however, difficult to realize adhesive layers without air bubbles/thickness irregularity for mass-produced two-substrate optical disks with a high yield.